Groves J T, Crowley S J, Shalyaev K V
Department of Chemistry, Princeton University, New Jersey 08544, USA.
Chirality. 1998;10(1-2):106-19.
Enantioselective catalytic epoxidation of olefins is an important problem from both practical and mechanistic points of view. The origins of chiral induction by asymmetric porphyrin and salen complexes were investigated by FT-NMR T1 relaxation techniques. A new chiral vaulted porphyrin (1) that carries (S)-binaphthyl-L-alanine straps across both faces of the porphyrin macrocycle was synthesized and characterized. (R)-styrene oxide was obtained in > 90% ee in the initial stages of styrene epoxidation with F5PhIO catalyzed by 1-Fe(III)Cl. The transition state for olefin epoxidation with high-valent metal-oxo species was modeled by coordinating epoxides to paramagnetic copper complexes of the corresponding ligands. The epoxide enantiomer that better fit the chiral cavity of the catalyst, as revealed by T1 relaxation measurements, was also the major product of catalytic olefin epoxidation. These results are consistent with the "lock-and-key" mechanism of asymmetric catalysis by metalloporphyrins. The copper complex of a chiral salen ligand showed no differentiation in terms of T1 relaxation rates between the enantiomers of cis-beta-methylstyrene oxide in contrast to the high enantioselectivity observed for catalytic epoxidation.
从实际应用和反应机理的角度来看，烯烃的对映选择性催化环氧化都是一个重要的问题。通过傅里叶变换核磁共振（FT-NMR）T1弛豫技术研究了不对称卟啉和席夫碱配合物的手性诱导起源。合成并表征了一种新型的手性拱形卟啉（1），该卟啉在卟啉大环的两个面上都带有（S）-联萘基-L-丙氨酸链。在用1-Fe（III）Cl催化F5PhIO对苯乙烯进行环氧化反应的初始阶段，得到了对映体过量（ee）>90%的（R）-环氧苯乙烷。通过将环氧化物与相应配体的顺磁性铜配合物配位，对高价金属-氧物种催化烯烃环氧化的过渡态进行了建模。T1弛豫测量结果表明，与催化剂手性腔更好匹配的环氧化物对映体也是催化烯烃环氧化的主要产物。这些结果与金属卟啉不对称催化的“锁钥”机理一致。与催化环氧化反应中观察到的高对映选择性相反，手性席夫碱配体的铜配合物在顺式-β-甲基环氧苯乙烷对映体的T1弛豫速率方面没有表现出差异。
